During sampling measurement of periodic signals, it is often desirable to be able to perform the measurement in a phase-locked manner. When the frequency of the periodic signal varies, as for example the frequency of an a.c. network, it may be difficult to fulfil the demand for phase locking, in which case problems with incorrect measurement and loss of accuracy may arise. The present invention comprises a method of adapting the sampling frequency to the periodicity of the periodic signal, so that phase locking can be maintained even through the frequency f.sub.m of the periodic signal varies.